1. Field of the Invention
The present invention relates to a display device, and more particularly to a display device which is referred to as an active matrix type display device.
2. Description of the Related Art
For example, a liquid crystal display device of this type is configured such that a pair of substrates are arranged to face each other with liquid crystal therebetween and, on a liquid-crystal-side surface of one of these substrates, g ate signal lines which extend in the x direction and are arranged in parallel in the y direction and drain signal lines which extend in the y direction and are arranged in parallel in the x direction are formed, wherein respective regions which are surrounded by these respective signal lines constitute pixel regions.
Then, each pixel region includes at least a thin film transistor which is driven in response to a scanning signal from the one-side gate signal line and a pixel electrode to which a video signal is supplied from the one-side drain signal line through the thin film transistor.
Further, on a liquid-crystal-side surface of one substrate or another substrate, counter electrodes which generate an electric field between the counter electrodes and the pixel electrodes and control the optical transmissivity of the liquid crystal are provided.
Still further, in recent years, there has been known a liquid crystal display device in which the thin film transistor has a semiconductor layer which is made of polysilicon (p-Si) and can be formed at a low temperature.
In the liquid crystal display device, with respect to the formation of a gate insulation film of the thin film transistor, the gate insulation film is usually formed by a method which can form the film at a relatively low temperature such as a CVD method, for example, while obviating a method which forms the film at a high temperature such as a high-temperature oxidation method.
However, the available gate insulation film is not always limited to the insulation film which is formed by the CVD method or the like. That is, there has been also known a gate insulation film having the multilayered structure which includes an insulation film formed by a low-temperature oxidation method or the like (Japanese Unexamined Patent Publication No. Hei8(1996)-195494, Japanese Unexamined Patent Publication No. 2000-91590, and Japanese Unexamined Patent Publication No. 2000-223712).
These constitutions are provided for reducing the interface level density of the thin film transistor or for obtaining other advantageous effects.
The liquid crystal display device having these thin film transistors formed of polysilicon is, usually, configured such that a scanning signal drive circuit which supplies scanning signals to gate signal lines or a video signal drive circuit which supplies video signals to drain signal lines and the like are formed on one substrate, while semiconductor layers of MIS (Metal Insulator Semiconductor) transistors which are incorporated into these peripheral circuits are also formed of polysilicon. This is because the MIS transistors can be formed along with the formation of the thin film transistors on pixel regions.